Fix docstring for scheduler_default_filters param

The docstring for the nova::scheduler::filter::scheduler_default_filters
paramater tells the user to specify it as a comma-separated string.
In actuality, the param is passed into stdlib's join() function, so
it's actually expecting an array.  This patch updates the documentation
accordingly.

Change-Id: Ib0b2757d4ddf0d9821aac8a1537e1338f69e1d69
Closes-bug: 1354477